Localized Corrosion of Stainless Steel in a Nuclear Waste Cooling Water System—Part 5: Inhibition Studies Using “All-Metal” Artificial Pits

Parts of a nuclear waste cooling water system, constructed from an austenitic stainless steel, are known to be susceptible to localized corrosion. To enable the investigation of corrosion inhibitors for potential application to that system, an artificial corrosion pit cell made from stainless steel was developed to simulate corrosion of large pits. A number of inhibition treatments were tested using these pit specimens, including substitution of chloride-containing water with demineralized water, sodium hydroxide dosing, and molybdate dosing. Corrosion was monitored by measuring the current flowing between the pit specimens and a large piece of non-corroding stainless steel wire mesh using a “zero-resistance” ammeter. The corrosion persisted despite long-term purging with high-quality demineralized water or sodium hydroxide dosing to pH 11. Sodium molybdate dosing to a molybdate concentration of 50 mg/L was ineffective, though it did have a strong effect of catalytically decomposing hydrogen peroxide (included to simulate the effects of water radiolysis). Two distinctly different internal morphologies were found consistent with salt-filming and the etch-type mechanisms of pitting.